Ebullient cooling of high temperature metalliferous vapors

ABSTRACT

Metalliferous vapors are condensed on an ebulliently cooled heat-conducting wall member of a heat exchanger. The metalliferous vapor comprising the vapors of an aluminum halide and an alkali metal or alkaline earth metal are condensed to produce two immiscible liquid phases of aluminum and an alkali metal, or alkaline earth metal, halide respectively. The temperature of the ebullient liquid is controlled by regulating the composition of the ebullient liquid, the pressure over it or both. Preferably the wall member is constructed of a material which is preferentially wet by the molten alkali metal or alkaline earth metal halide relative to molten aluminum. Accordingly, erosion of the salt-wet wall member by liquid aluminum is substantially reduced. Preferred ebullient liquids include alloys of copper and magnesium or zinc and magnesium chloride. Any of these materials would safely mix with liquid aluminum and the alkali metal or alkaline earth metal condensate in the event of catastropic failure of the heat exchanger walls.

United States Patent Layne 51 Nov. 27, 1973 EBULLIENT COOLING OF HIGHOTHER PUBLICATIONS TEMPERATURE METALLIFEROUS VAPORS Gemnger Handbook ofHeat Transfer Media TP363G4 Reinhold Pub. Corp., NY. 1962, pp, [75]Inventor: Gilbert S. Layne, Midland, Mich. 208-210,

[73] Assignee: The Dow Chemical Company,

Midland, Mich Primary Examiner-L. Dewayne Rutledge AssistantExaminer--M. .l. Andrews [22] Flled: 27, 1971 Attorney-William R. Norriset al. [21] Appl. No.: 212,728

Related US. Application Data [57] ABSTRACT [63] Continuation of Sen N0851 Jan. 6 1970 Metalliferous vapors are condensed on an ebullientlyabandoned cooled heat-conducting wall member of a heat exchanger. Themetalliferous vapor comprising the va- 52 U.S. c1 75/68 B of an aluminumhalide and an alkali metal [51] Int. Cl c221 21/00 earth P are ndensedPmdu 58 Field of Search 75/68 B, 68 R; misc'ble quld Phases aluminum andan alkali 423/490 metal, or alkaline earth metal, halide respectively.The temperature of the ebullient liquid is controlled by [56] ReferencesCited regulating the comtposliticn (Ff till: elaullienltl liquid), thepressure over it or 0t re era yt e wa mem er 1s UNITED STATES PATENTSconstructed of a material which is preferentially wet 3,397,056 8/l968Layne et al. 75/68 R the molten metal or alkaline earth metal ha.1,666,251 4/1928 Andrews 266/15 X lide relative to molten aluminum.Accordingly, ero- Zig ag? sion of the salt-wet wall member by liquidaluminum is 3243282 M966 McGeer 75/68 B substantially reduced. Preferredebullient liquids in- 2:724:644 11 1955 Mathieu 75/68 R x dude alloyscopper and magnesum Zinc and 3,484,233 12/1969 Bonilla 75/63 magnesiumchloridey of these materials would 2,019,245 10/1935 Berry 165/111safely mix with liquid aluminum and the alkali metal 2,914,398 11/1959Johnston et a1 75/68 B or alkaline earth metal condensate in the eventof FOREIGN PATENTS OR APPLICATIONS Great Britain 75/68 catastropicfailure of the heat exchanger walls.

8 Claims, 2 Drawing Figures 1 Nov. 27, 1973 United States Patent [191Layne EBULLIENT COOLING OF HIGH TEMPERATURE METALLIFEROUS VAPORS This isa continuation of application Ser. No. 851, filed Jan. 6, 1970 and nowabandoned.

A method for the recovery of aluminum from impure sources of the metalis described in U.S. Pat. No. 3,397,056. In one phase of the processtherein described, a mixture of aluminum halide vapor and an alkalimetal, or alkaline earth metal, vapor is condensed to recover a purifiedaluminum and reconstituted alkali metal or alkaline earth metal halide.Although such a condensation reaction is readily accomplished on anyheat transferring surface below the dew point of the vapors, seriousproblems of erosion and strength deterioration occur in most materialsof construction after prolonged used.

In addition, because of the high temperatures and heat fluxes involved,satisfactory large scale condensers must be capable of rapidly removingheat under highly variable conditions of operation.

In view of the above desiderata of the art, it would be advantageous,and it is a particular object of the instant invention to provide a newmethod for condensing metalliferous vapor systems as herein defined,i.e., AlX M representing respectively aluminum halide and an alkalimetal or alkaline earth metal vapor.

A particular object is to provide an improved method for condensing thedescribed metalliferous vapors with the capability of removing largeheat fluxes under conditions of varying vapor temperature and mass flowrates.

Another object is to provide an improved method for condensing themetalliferous vapors whereby erosion of heat exchanger constructionmaterials is minimized and equipment life is extended.

A still further object is to provide a condensing system for thedescribed metalliferous vapors in which the temperature of the coolingsurface is regulated. A particular object in the latter regard is toprovide an im proved cooling system in which the composition of anebullient liquid is regulated to adjust cooling temperatures.

Another object in the latter regard is to provide an ebullient coolingmethod in which the temperature is regulated by the pressure maintainedon the ebullient liquid.

Pursuant to the above objectives, and in accordance with the instantinvention, an improved process for cooling metalliferous vaporscomprises the following steps: Metalliferous vapors are contacted with aheat exchanging surface of a wall member of a heat exchanger, which atleast partially defines wall encloses a condensation zone. A secondsurface of the will member is cooled with an ebullient liquid. Thelatter is characterized by boiling temperature below the condensationtemperature of the metalliferous vapors but above the melting point ofaluminum. Heat absorbed by the ebullient liquid is transmitted by vaporsof the ebullient liquid to a secondary heat exchanging means. The vaporsof the ebullient liquid are thus condensed and returned to the body ofebullient liquid.

Metalliferous vapors to which the invention is generally applicable arecharacterized by the symbols AlX M wherein AlX is one or more aluminumhaldides, e.g., aluminum monofluoride, aluminum trifluoride, aluminummonochloride, aluminum trichloride and M is a metal of Groups I and II,e.g., an alkali metal and r 2 alkaline earth metals. In a preferredembodiment, the invention is applied to a vapor system comprisingaluminum monofluoride and magnesium, which upon condensing producesaluminum and magnesium fluoride as liquid. This reaction is furtherelaborated in U.S. Pat. No. 3,397,056.

It is an essential characteristic of the foregoing method that thetemperature of the condensing surface be controlled within limits, whichon the upside are sufficiently low to achieve the desired condenstationand heat removals from incoming vapors and which at the lower end aresufficiently high to be above the melting point of the condensateproducts, i.e., aluminum and an alkali metal or alkaline earth metalhalide. Accordingly, temperature control in the ebullient fluid is anessential and most significant aspect of the instant invention. In thedrawings:

FIG. 1 is a schematic illustration of the application of ebullientcooling in accordance with the instant invention. A special modificationof the apparatus equalizes pressure within the ebullient system withthat in the condensing zone.

FIG. 2 depicts a condensing, tube-type, heatexchanger with ebullientliquid within the tubes and means for continuous aluminum and saltby-product take off.

By reference to FIG. 1, the process of the instant invention isillustrated by condensing aluminum fluoride and magnesium vapors withina condensation zone 10 defined by condenser housing 11 having a vaporinlet 27., Within the product condenser housing 11 is an ebullientliquid housing 12 having a condensing surface 13.? Heat is absorbed bythe ebullient liquid 16 thereby condensing aluminum and magnesiumfluoride 15 on the condensing surface 13. The condensing surface 13 ispreferably made of a material which is preferentially wet by themagnesium fluoride relative to liquid aluminum. The condensed liquidaluminum and magnesium flupride l5 drain through downpipe 28 into aproduct coalescing vessel 20, in which they separate by gravity intoessentially pure liquid phases of aluminum 25 and magnesium fluoride 24.Each of these products is withdrawn, either continuously orintermittently, as they are accumulated, through outlets 22 and 23,respectively.

Vapors from the ebullient liquid 16 rise in vapor zone 9 to contact theheat exchanging coil 14 where they are condensed and ultimately returnedto the body of the ebullient liquid 16. The vapor zone 9 above theebullient liquid communicates by means of conduit 18 with the vapor zoneabove the condensate in collection vessel120. Within conduit 18 is anon-off valve 17 and a gas discharge vent with valve 19. An inert gas 31is connected to the conduit 18 through pressure valve 29.

In the preferred mode of operation, the system is operated at a givenpressure, which is equilibrated in zones 9 and 10 by maintaining valve17 in an open position and valve 29 closed. Pressure regulating valve 19is set to control at the desired operating pressure. This systemprovides direct pressure communication between the ebullient boilingvapor zone 9 and the vapor zo e 10 of the product condenser housing 11.In the event the noncondensible gas pressure over the ebullient liquidsignificantly exceeds that within the condenser housing II, the pressuredifferential will be reduced by pressure release into the condenserhousing 11 through the condenser downpipe 28. Alternatively,

should pressure within the condenser housing 11 exceed that in zone 9,the pressure flow will reverse. The continuous withdraw] of aluminum andmagnesium fluoride from the condensate collection vessel assures desiredliquid levels of these two products.

A principal benefit of operating in the foregoing manner is that thepressure within the ebullient liquid housing is equalized with thatwithin the condenser housing. This minimizes stresss on the heatconducting wall members of the ebullient liquid housing 12.

In an optional variation, valve 17 may be closed and tempeatureregulation of the ebullient liquid 16 achieved by controlling thenoncondensible gas pressure in vapor zone 9. This is achieved byregulating the supply of inert gas 31 by means of pressure regulatingvalve 29 and pressure regulating gas vent 32. Whenever a lower boilingtemperature is required in the ebullient liquid, as for example when theheat flux entering the condenser is high, the temperature of theebullient liquid is lowered by releasing pressure in the vapor zone 9through valve 32. When a higher temperature in vapor zone 9 isnecessitated by a lower heat flux, valve 32 is adjusted to regulate at ahigh pressure and an inert gas supply is administered to the vapor zone9 through valve 29.

FIG. 2 illustrates another embodiment of the invention in which aluminumhalide and metal vapors enter a condenser housing 40 through vapor inlet42. Within the housing is a tube bundle 51 of individual tubes 52 fittedinto tube headers 50 and 54. The tube bundle 51 defines, in conjunctionwith the interior walls of the condenser housing 40, a condensation zone41. This zone is occupied, during operation with incoming vapors andcondensed liquid phases of aluminum 61 and metal halide 60, The liquidaluminum 61 is discharged through outlet 46 containing a trap equippedwith a noncondensible gas vent 45. The metal halide 60 is discharged asa liquid through discharge port 44. Below the tube bundle header 50 islocated a reservoir 49 of an ebullient liquid 65a, which can be, forexample, a copper-magnesium alloy. Within this reservoir 49 are aspecial resistance heating means 47, 47a to maintain the ebullientliquid above its liquidus temperature. In the lower portion of the heatexchanger housing 40 there is also a drain 48 from which accumulationsof non-volatile residuals may be removed periodically from the reservoir49.

The upper tube header 54 defines in conjunction with the condenserhousing 41 a vapor zone 63 above which there are condensing means 53 forcondensing the vapors 65a of the boiling ebullient liquid 65b. Thecondensate 65d returns to the boiling ebullient liquid. The illustratedcondensing means 53 is simply a steam cooled tube heat exchanger.

The vapor zone 63 is adapted at port 56 for the removal of vapors of theebullient liquid. These vapors may be condensed by means not shown andrecycled at port 55.

In one mode of operation, temperature control is achieved by regulationof pressure on the ebullient liquid. By either introducing orwithdrawing a noncondensible gas over the ebullient liquid, its boilingtemperature may be increased or decreased as is necessary to maintainthe condensing surface at a suitable temperature.

Although pressure control is a feasible mode for regulating thetemperature of an ebullient system and thus the temperature of thecondensing surface of the present invention, the engineering ofsatisfactory equipment in which to carry out the process is complicatedby the requirement of high strengths at high temperatures inconstruction materials.

It is a preferred mode of operation to employ, as the ebullient liquid,a material which is characterized by the desired boiling temperature forebullient cooling or is readily adjustable to provide such a boilingtemperature. In the latter regard, a satisfactory ebullient liquidcomprises two materials having significantly different vapor pressuresand at least one of these materials boils at a temperature below thatnecessary to achieve condensation in the instant invention. In such asystem, the more volatile component vaporizes preferentially to the lessvolatile component. If the entire condensate of such vapors is returnedto the body of ebullient liquid, an equilibrium is quickly establishedand the temperature of ebullient liquid controlled to a constant leveldetermined by the composititon of the liquid.

In the event increases in the boiling temperature of the ebullientliquid are desired, a portion of the vapors is condensed and withdrawnfrom the boiling system thereby relatively increasing the proportion ofthe higher boiling component and thus the temperature of the ebullientliquid. Similarly, the introduction into the body of ebullient liquid ofadditional amounts of a lower boiling ebullient liquid component reducesthe temperature on the system.

Alloy mixture of the volatile metals such as magnesium, zinc and bismuthwith one or more less volatile or stable metals such as copper, lead,tin or silver are suitable for use as an ebullient liquid in theaforedescribed controlled system. Salt mixtures which can be similarlyemployed include, for example, mixtures of volatile salts such asmagnesium chloride, lead chloride and sodium chloride with less volatilesalts such as calcium chloride, calcium fluoride and magnesium fluoride.

Utilizing the foregoing, a method is provided for condensing themetalliferous vapors described above in which heat flux is controlled bythe temperature differential across the heat exchanging wall and thustemperatures are maintained within necessary and desirable limits.

In the practice of the invention, the heat-conducting wall member of theheat exchanger is either manufactured of, or surface clad with, amaterial which is wetted preferentially by an alkali metal or alkalineearth metal salt relative to liquid, molten aluminum. This preferentialwetting is important to the ultimate protection of the heat exchangersurface from molten aluminum which is extremely corrosive. Materials ofconstruction possessing the desired wetability characteristics withrespect to the two phase liquid-liquid condensates produced inaccordance with the invention include the borides, nitrides, andcarbides of aluminum, silicon, titanium and zirconium and boron carbideor nitride. Examples are graphite, aluminum nitride, aluminum carbide,zirconium carbide, titanium carbide, silicon carbide and boron nitride.

In a special embodiment, the ebullient fluid is selected of a materialhaving a desired boiling point and this material is purified bycontinuous distillation. In such event, the material is substituted forthe ebullient liquid and the distillation allowed to proceed with therecovery of condensate to produce the desired purified product.Illustratively, magnesium chloride boils at a temperature which isgenerally suitable for cooling the inside surface of the heat conductingwall member. Accordingly, it is possible to continuously distill andpurify magnesium chloride, while condensing the metalliferous vapors toproduce aluminum and magnesium fluoride according to the teachings ofU.S. Pat. No. 3,397,056.

What is claimed is: 1. A process which comprises 1. contactingcondensible AlF Mg vapors, with a first surface of a heat conductingwall member in a condensation zone to produce a condensate containingdistinct molten phases of aluminum and magnesium fluoride, 2. cooling asecond surface of the wall member with an ebullient liquid in heatexchange relation with the wall member, said liquid characterized by aboiling temperature, under extant conditions, below the condensationtemperature of the metalliferous vapors in the condensation zone butabove the melting point of aluminum and said ebullient liquid beingselected from the group consisting of magnesium chloride or a moltenalloy of magnesium, bismuth or zinc with one or more members of thegroup consisting of copper, lead, tin and silver. 2. A method as inclaim 1 wherein the first surface of the heat conducting wall member ispreferentially wet by the magnesium fluoride relative to moltenaluminum.

3. A method as in claim 1 and including the additional step ofincreasing or decreasing the pressure on the ebullient liquid tocorrespondingly increase or decrease its boiling temperature.

4. A method as in claim 3 wherein the extent of pressure increase ordecrease is controlled relative to the amount of incoming vapors tomaintain the heat flux across the heat conducting wall member at a levelsufficient to condense the AIP Mg vapors, but without solidifying eithercondensate phase.

5. A method as in claim 1 wherein the ebullient liquid is a compositioncomprising two materials having significantly different boilingtemperatures, at least one of these materials being furthercharacterized by having a boiling temperature below the condensationtemperature of the vapors and both materials characterized by having amelting point below such condensation temperature.

6. A method as in claim 5 wherein the ebullient liquid is a mixture ofmagnesium and copper.

7. A method as in claim 5 wherein the temperature of the ebullientliquid is controlled to a desired level by adjusting the relative ratioof the two materials in the ebullient liquid.

8. A method as in claim 7 wherein the ratio is adjusted by distillingoff the more volatile component from, or returning the same to, theebullient liquid.

2. A method as in claim 1 wherein the first surface of the heatconducting wall member is preferentially wet by the magnesium fluoriderelative to molten aluminum.
 2. cooling a second surface of the wallmember with an ebullient liquid in heat exchange relation with the wallmember, said liquid characterized by a boiling temperature, under extantconditions, below the condensation temperature of the metalliferousvapors in the condensation zone but above the melting point of aluminumand said ebullient liquid being selected from the group consisting ofmagnesium chloride or a molten alloy of magnesium, bismuth or zinc withone or more members of the group consisting of copper, lead, tin andsilver.
 3. A method as in claim 1 and including the additional step ofincreasing or decreasing the pressure on the ebullient liquid tocorrespondingly increase or decrease its boiling temperature.
 4. Amethod as in claim 3 wherein the extent of pressure increase or decreaseis controlled relative to the amount of incoming vapors to maintain theheat flux across the heat conducting wall member at a level sufficientto condense the A1F + Mg vapors, but without solidifying eithercondensate phase.
 5. A method as in claim 1 wherein the ebullient liquidis a composition comprising two materials having significantly differentboiling temperatures, at least one of these materials being furthercharacterized by having a boiling temperature below the condensationtemperature of the vapors and both materials characterized by having amelting point below such condensation temperature.
 6. A method as inclaim 5 wherein the ebullient liquid is a mixture of magnesium andcopper.
 7. A method as in claim 5 wherein the temperature of theebullient liquid is controlled to a desired level by adjusting therelative ratio of the two materials in the ebullient liquid.
 8. A methodas in claim 7 wherein the ratio is adjusted by distilling off the morevolatile component from, or returning the same to, the ebullient liquid.